In a subsea well of the type concerned herein, a wellhead housing locates on the sea floor. Strings of casing extend into the well, with the casings being supported in the wellhead housing. A casing hanger seal is installed between the casing hanger at the upper end of the casing and the wall of the wellhead housing. The operator installs the casing and the seal remotely and sometimes in seas of considerable depths.
There have been a number of types of running tools used and proposed in the patented art. With the advent of metal-to-metal casing hanger seals, the forces required to set these seals are greater than the prior art elastomeric seals. Running tools have to be capable of delivering very large forces. One type utilizes hydraulic pressure, as shown in U.S. Pat. Nos. 4,969,516 and 4,928,769. The hydraulic pressure is generated by axial movement of the drill string, which moves a piston within a sealed hydraulic chamber in the running tool. These hydraulic tools work well. However, they are complex and expensive.
U.S. Pat. No. 5,044,442 shows a type that is hydraulically actuated, but uses annulus pressure. Rams are closed around the drill string, creating a chamber located above the wellhead housing within the riser. A bulk seal seals a portion of the running tool to the wellhead housing above the setting sleeve and casing hanger seal. The bulk seal enables pressure to be applied to a piston of the running tool. Fluid is pumped down a choke and kill line to this chamber, which actuates the piston within the running tool to set the casing hanger seal. The annulus pressure actuated hydraulic tool described in that patent is feasible, however a possibility exists that the bulk seal could seal on the wellhead housing at a point above the desired position. If so, the casing hanger seal might be actuated before it is located fully within the pocket between the casing hanger and the bore of the wellhead housing.
Subsea drilling is a problem in certain areas, such as the Gulf of Mexico. Shallow formations in the Gulf of Mexico present special problems that must be solved with a variety of techniques, which include using extra casing strings, etc. Another solution proposed is drilling with positive pressure. This may require the use of a rotating drilling head, seals and drill pipe. The prior art only used this equipment for horizontal or underbalanced wells at the surface, not subsea.